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Formaldehyde smell

I've just converted my charcoal furnace to propane / air - I tested it out today for the first time, cranked up the gas and air blast and got a choking formaldehyde smell from it. I'm assuming that this was due to decomposition of the propane by incomplete burning. Not enough air from my blower ? - the flame was yellow. Is it a blue flame I should be aiming for ?

I don't know what formaldehyde smells like. Maybe it's residue in your furnace or perhaps it's coming from the refractory. Charcoal and coal (you mentioned anthracite in your other post today) burn insanely hot but a propane furnace sets up at least somewhat different dynamics that may be burning tar or some part of your refractory that was previously unaffected. I don't know that burning propane creates an odor other than what that rotten egg additive they put in it might produce. I'll be interested to know what others think.

Thanks for replying - Formaldehyde is also known as formalin when in solution. It's used for preserving biological specimens. I know the smell well from my days of working in a lab. It's the same kind of smell I've detected when my kitchen hob burner hasn't properly lit all around the burner cup. The refractory is ordinary fire cement and has been through over 30 melting sessions using charcoal / anthracite.
As the optimum ratio is apparently 1:30 (gas/air), the problem might be that I can't match the gas flow with enough air on high settings. It was a terrible stench - burned the nose and made the eyes water. We couldn't tolerate it for more than a few seconds. I'm going to do a test run today with minimal gas feed and plenty of air, tempering up a new crucible, so we'll see what happens then.